Transmission and hydraulic control system

ABSTRACT

A hydraulic control system for an automatic transmission with a torque converter includes two regulator valves controlled by a single variable force solenoid (VFS). A bypass clutch regulator valve increases the pressure to a bypass clutch apply chamber as the VFS pressure increases. A converter charge regulator valve decreases the pressure in a converter charge circuit as the VFS pressure increases. The converter charge circuit is in series with a lubrication circuit. An orifice restricts the flow through these circuits such that they can be supplied from the line pressure circuit rather than a lower priority circuit. In one embodiment, an on/off solenoid opens a flow control valve to bypass the orifice when additional flow is required. In another embodiment, an electric pump supplements the flow in these circuits when required. This later embodiment includes a switch valve such that the electric pump also supports stop/start operation.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. provisional application Ser.No. 62/012,786 filed Jun. 16, 2014, the disclosure of which is herebyincorporated in its entirety by reference herein.

TECHNICAL FIELD

This disclosure relates to the field of automatic transmissions formotor vehicles. More particularly, the disclosure relates to a hydrauliccontrol system for a transmission having a torque converter and a bypassclutch.

BACKGROUND

Many vehicles are used over a wide range of vehicle speeds, includingboth forward and reverse movement. Some types of engines, however, arecapable of operating efficiently only within a narrow range of speeds.Consequently, transmissions capable of efficiently transmitting power ata variety of speed ratios are frequently employed. When the vehicle isat low speed, the transmission is usually operated at a high speed ratiosuch that it multiplies the engine torque for improved acceleration. Athigh vehicle speed, operating the transmission at a low speed ratiopermits an engine speed associated with quiet, fuel efficient cruising.

FIG. 1 depicts a typical front wheel drive powertrain arrangement. Flowof mechanical power is shown by solid lines. Power is provided byinternal combustion engine 10. Transmission input shaft 12 transmitspower from the crankshaft of engine 10 to torque converter 14. Torqueconverter 14 permits the engine to idle while the vehicle is stationary.Torque converter 14 transmits the power to gearbox 16 via turbine shaft18. In some operating conditions, torque converter 14 may decreasesshaft speed and increase shaft torque. Gearbox 16 adjusts the speed andtorque according to current vehicle requirements and transmits the powerto differential 20. Differential 20 transmits the power to left andright wheels 22 and 24 while permitting slight speed differences whenthe vehicle turns a corner.

Torque converter 14 requires a supply of fluid to establish ahydrodynamic torque flow path and may also require a supply ofpressurized fluid to engage a lock-up clutch. Gearbox 16 requires asupply of fluid for lubrication and cooling. The speed ratio of gearbox16 is controlled by supplying fluid at controlled pressures to acollection of shift elements such as brakes and clutches. The fluid forthese purposes is supplied by pump 26 and regulated by valve body 28.Pump 26 is mechanically driven by the transmission input shaft 12. Pump26 draws fluid from a sump 30 and provides the fluid to valve body 28 atan elevated pressure called line pressure. Valve body 28 distributes thefluid into a number of separate circuits directed to torque converter 14and gearbox 16 at various pressures that may be controlled to pressuresless than line pressure. In some cases, the valve body directs flowthrough a cooler 32. Fluid drains from the gearbox and valve body backto the sump 30. Components within the dotted box 34 are typicallycontained within a common housing and called a transaxle.

SUMMARY OF THE DISCLOSURE

A transmission includes a torque converter, a bypass clutch, and ahydraulic control system having a converter charge circuit, a converterreturn circuit, a bypass clutch apply circuit, and a lubricationcircuit. Pressure in the bypass clutch apply circuit and in theconverter charge circuit are controlled by a bypass clutch regulatorvalve and a converter charge regulator valve, respectively. The bypassclutch regulator valve and the converter charge regulator valve respondto a common variable force solenoid (VFS). In some embodiments, thebypass clutch may define a balance chamber supplied with fluid from thelubrication circuit. The converter charge circuit, which is arranged inseries with the converter return circuit and the lubrication circuit, issupplied via the converter charge regulator valve from a line pressurecircuit. An orifice restricts the flow in the converter charge circuit,preventing the need to feed these circuits from a lower prioritycircuit. Alternative mechanisms may increase the flow in the convertercharge circuit and the lubrication circuit when necessary. In oneembodiment, a flow control valve activated by an on/off solenoidbypasses the orifice to increase flow. In another embodiment, anelectric pump provides additional flow when necessary.

In another embodiment, a line pressure circuit is supplied by both afirst pump driven by the transmission input shaft and by a second pumpdriven by an electric motor. A switch valve directs the flow of theelectric pump either to the line pressure circuit or to a torqueconverter charge circuit, based on an on/off solenoid. First and secondregulator valves may adjust the pressures in a torque converter chargecircuit and a bypass clutch apply circuit, respectively, based on firstVFS. A third regulator valve may adjust the pressure in the linepressure circuit based on a second VFS.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a vehicle powertrain.

FIG. 2 is a schematic diagram of a torque converter assembly.

FIG. 3 is a cross sectional view of a bypass clutch.

FIG. 4 is a schematic diagram of a portion of a first hydraulic controlsystem.

FIG. 5 is a graph of two controlled pressures in the hydraulic controlsystem of FIG. 4 with respect to a VFS pressure.

FIG. 6 is a schematic diagram of a portion of a second hydraulic controlsystem.

DETAILED DESCRIPTION

Embodiments of the present disclosure are described herein. It is to beunderstood, however, that the disclosed embodiments are merely examplesand other embodiments can take various and alternative forms. Thefigures are not necessarily to scale; some features could be exaggeratedor minimized to show details of particular components. Therefore,specific structural and functional details disclosed herein are not tobe interpreted as limiting, but merely as a representative basis forteaching one skilled in the art to variously employ the presentinvention. As those of ordinary skill in the art will understand,various features illustrated and described with reference to any one ofthe figures can be combined with features illustrated in one or moreother figures to produce embodiments that are not explicitly illustratedor described. The combinations of features illustrated providerepresentative embodiments for typical applications. Variouscombinations and modifications of the features consistent with theteachings of this disclosure, however, could be desired for particularapplications or implementations.

FIG. 2 schematically depicts torque converter 14. A stationary frontsupport 40 is fixed to the transaxle housing. Transmission input shaft12 drives impeller 42. Turbine 44 drives turbine shaft 18. Stator 46 isconnected to front support 40 via one way clutch 48. Bypass clutch 50selectively couples transmission input shaft 12 to turbine shaft 18.Pump drive sprocket 52 is fixed to input shaft 12. Impeller 42, turbine44, and stator 46 define a torque converter cavity filled with fluid.

Vanes on impeller 42 tend to propel the fluid outwardly in response torotation of impeller 42. The fluid then exerts torque on vanes ofturbine 44 as the fluid circulates inwardly. Vanes on stator 46 redirectthe fluid exiting the turbine back into the impeller. Whenever impeller42 is rotating faster than turbine 44, positive torque is exerted onturbine 44 and resistance torque is exerted on impeller 42. Themagnitude of the torque depends upon the relative speeds of the impellerand turbine. Due to the action of the stator, the torque on the turbinemay be higher than the resistance torque on the impeller. As the turbinespeed approaches the impeller speed, one way clutch 48 overruns allowingstator 46 to rotate.

When bypass clutch 50 is engaged, power is transferred from transmissioninput shaft 12 to turbine shaft 18 through bypass clutch 50 as opposedto the hydrodynamic power transfer path described above. Because thispower transfer path is more efficient than the hydrodynamic powertransfer path, it is preferred during steady state cruising situations.However, the hydrodynamic power flow path provides torque multiplicationand vibration isolation that make it preferable in other circumstancessuch as during low speed driving and while gearbox 16 is shifting fromone speed ratio to another. The torque capacity of bypass clutch 50 maybe regulated such that some power is transferred by each power transferpath.

FIG. 3 shows the structure of bypass clutch 50. A set of friction plates60 are splined to transmission input shaft 12 such that they are forcedto rotate with input shaft 12 but can move axially. A snap ring 62limits the axial movement toward the left. The friction plates 60 areinterleaved with a set of separator plates 64 that are splined toturbine shaft 18. Piston 66 is supported to slide axially with respectto input shaft 12. When pressurized fluid is routed to chamber 68,piston 66 forces the friction plates and separator plates together.Friction between the friction plates and separator plates transmitstorque between transmission input shaft 12 and turbine shaft 18. Thetorque capacity of the clutch is controlled by varying the pressure inapply chamber 68. The pressurized fluid may be routed to apply chamber68 through a channel 70 in turbine shaft 18 and a channel 72 in inputshaft 12. The fluid is directed between channels 70 and 72 by a seals 74and 76. When fluid pressure is removed, return spring 78 forces thepiston to the right to release the clutch.

As input shaft 12 rotates, centrifugal forces cause the pressure inchamber 68 to exceed the pressure in channel 70. Since these forcesfluctuate based on input shaft speed, the variations make it difficultto accurately control the torque capacity. To compensate for thesefluctuations, fluid at low pressure is routed to balance chamber 80. Thefluid may be routed through a channel 82 in turbine shaft 18 and achannel 84 in input shaft 12. Channels 72 and 84 are at differentcircumferential locations within turbine shaft 18 such that they do notintersect with one another. Seal 74 directs the flow from channel 82 tochannel 84. Balance chamber 80 is designed to have nearly the same areaacting on piston 66 as apply chamber 68 and at nearly the same radiisuch that the centrifugal forces generated in the two chambers cancelone another out.

In an alternative embodiment, fluid at moderate pressure may be routedto balance chamber 80. For example, the fluid circuit used to providefluid to the torque converter cavity defined by the turbine, impeller,and stator, called the converter charge circuit, may be routed tobalance chamber 80. This eliminates the need for return spring 78because the moderate pressure forces piston 66 to the right whenpressure in apply chamber 68 is reduced to a low value. However, thetorque capacity is now a function of the difference in pressure betweenthe fluid provided to apply chamber 68 and the fluid provided to balancechamber 80 which can make control difficult in circumstances in whichthe converter charge pressure fluctuates.

FIG. 4 is a schematic diagram of a portion of a hydraulic networksuitable for use with the torque converter of FIG. 2 and the bypassclutch of FIG. 3. Pump 26 draws fluid from sump 30 and provides thefluid to line pressure circuit 90. The fluid may be drawn through afilter to remove impurities. Regulator valve 92 exhaust a portion of theflow back to the pump inlet port (or alternatively to sump 30) through avalve opening. Regulator valve 92 adjusts the size of the valve openingas necessary to maintain a desired pressure in line pressure circuit 90.A controller may adjust the desired pressure by commanding an electricalcurrent to Variable Force Solenoid (VFS) 94. Pump 26 may be a variabledisplacement pump in which case the controller may also adjust the linepressure by commanding a change in pump displacement. This has theadvantage of reducing the drag torque of pump 26 when lower pressure isrequired.

Bypass clutch regulator valve 96 controls the pressure in bypass clutchcircuit 98. The bypass clutch circuit 98 is routed to the bypass clutchapply chamber through passage 70 as described above. Valve 100 controlsthe pressure in the converter charge circuit 102. Valves 96 and 100control the pressure to a commanded value less than line pressure byadjusting the sizes of respective valve openings between the linepressure circuit and the respective output circuit such that thepressure drop across the valve openings result in the desired outputpressures. Since less pressure is required in the converter chargecircuit when the bypass clutch is engaged, these two valves can becontrolled by a common VFS 104. As shown in FIG. 5, the VFS generates acontrol pressure ranging from 0 to about 70 psi based on an electricalcurrent from a controller. In response, bypass clutch regulator valve 96controls the pressure in bypass clutch apply circuit 98 between 0 andabout 160 psi, with the commanded pressure increasing as the VFSpressure increases. At the same time, converter charge regulator valve100 controls the pressure in converter charge circuit 102 between about100 psi and about 45 psi, with the commanded pressure decreasing as theVFS pressure increases. These values are merely exemplary and may varydepending upon system parameters. In some instances, non-linearfunctions may be desirable. If converter charge is being routed to thebalance chamber 80, then it may also be routed to valve 96 so that valve96 can adjust the pressure in the bypass clutch circuit 98 such that thedifferential pressure acting on the bypass clutch piston is a directfunction of the VFS pressure.

Fluid returns from the torque converter chamber in return circuit 106.This circuit is routed through cooler 108 to lube circuit 110. Lubecircuit 110 is routed through channel 82 to balance chamber 80. It isalso routed to balance chambers of rotating clutches in gearbox 16, tovarious places in gearbox 16 to provide lubrication for the gears, andthrough the clutch packs of each of the shift elements in gearbox 16 toprovide cooling. A cooler bypass valve, not shown, may divert the fluidaround the cooler when the fluid is cold. The lube flow eventuallydrains back to sump 30 where it is recirculated by pump 26. Orifice 112is sized to restrict the amount of flow in these circuits to the amountneeded to dissipate heat and provide lubrication during normal operatingconditions. Excessive flow in this circuit increases drag because thepump must provide more fluid and because the fluid causes windage dragas it returns to the sump. When additional cooling flow is required, acontroller commands current to on/off solenoid 114. In response, flowcontrol valve 116 opens allowing additional flow.

In some prior art control systems, converter charge circuit 102 issupplied via a low priority circuit as opposed to a line pressurecircuit in order to ensure that adequate flow is available in the linepressure circuit for high demand events such as stroking an oncomingclutch. This could result in pressure variability in the convertercharge circuit during such events. Such variability makes control ofbypass clutch 50 difficult. The ability to restrict the flow using flowcontrol valve 116 permits converter charge circuit 102 to be suppliedvia the line pressure circuit so the converter charge pressure remainsstable during high flow demand events.

FIG. 6 is a schematic diagram of a second hydraulic network suitable foruse with the torque converter of FIG. 2 and the bypass clutch of FIG. 3.Items that are common with the hydraulic network of FIG. 4 have a commonreference number. In addition to engine driven pump 26, a second pump120 driven by a dedicated electrical motor is provided. Electric pump120 draws fluid from the same sump 30 as pump 26. Switch valve 122controls where the outlet of pump 120 flows based on whether or not thecontroller commands current to on/off solenoid 124. Orifice 126 is sizedto allow sufficient cooling and lubrication flow in normal drivingconditions. When additional cooling or lubrication flow is needed,switch valve 122 is commanded to direct flow from electric pump 120 tothe converter charge circuit 112 which results in increased flow in theconverter return circuit 106 and the lube circuit 110. Check ball 128prevents fluid from back-driving pump 120.

To increase fuel economy, some vehicles are designed to automaticallyshut the engine off when the vehicle is stationary such as while waitingat a traffic light. When the engine is off, engine driven pump 26 doesnot provide any fluid flow. If no provision is made for thiscircumstance, shift elements in the gearbox would lose capacity and thetransmission would be in neutral. When the driver releases the brakepedal and presses the accelerator pedal, the gearbox must be in gear inorder to provide acceleration. To keep the gearbox in gear, thecontroller commands switch valve 122 to provide flow from electric pump120 to the line pressure circuit 90.

While exemplary embodiments are described above, it is not intended thatthese embodiments describe all possible forms encompassed by the claims.The words used in the specification are words of description rather thanlimitation, and it is understood that various changes can be madewithout departing from the spirit and scope of the disclosure. Aspreviously described, the features of various embodiments can becombined to form further embodiments of the invention that may not beexplicitly described or illustrated. While various embodiments couldhave been described as providing advantages or being preferred overother embodiments or prior art implementations with respect to one ormore desired characteristics, those of ordinary skill in the artrecognize that one or more features or characteristics can becompromised to achieve desired overall system attributes, which dependon the specific application and implementation. As such, embodimentsdescribed as less desirable than other embodiments or prior artimplementations with respect to one or more characteristics are notoutside the scope of the disclosure and can be desirable for particularapplications.

What is claimed is:
 1. A transmission comprising: a torque converterhaving an impeller, a turbine, and a stator and defining a converterchamber; a bypass clutch configured to selectively couple the impellerto the turbine in response to a hydraulic pressure difference between anapply chamber and a balance chamber; a hydraulic control system having aconverter charge circuit to supply fluid to the converter chamber, aconverter return circuit to receive fluid from the converter chamber, abypass clutch apply circuit in fluid communication with the applychamber, and a lubrication circuit in series with the converter returncircuit; a bypass clutch regulator valve configured to adjust a pressurein the bypass clutch apply circuit in response to a current to avariable force solenoid; and a converter charge regulator valveconfigured to adjust a pressure in the converter charge circuit inresponse to the current to the variable force solenoid.
 2. Thetransmission of claim 1 wherein the lubrication circuit is in fluidcommunication with the balance chamber.
 3. The transmission of claim 1further comprising a flow control valve configured to adjust a flow ratein the converter return circuit in response to a current to an on/offsolenoid.
 4. The transmission of claim 1 further comprising: anelectrically driven pump; and a switch valve configured to alternatelydirect an output of the electric pump to the converter charge circuit orto a line pressure circuit in response to a current to an on/offsolenoid.
 5. The transmission of claim 1 wherein the converter chargeregulator valve adjusts the pressure in the converter charge circuit byadjusting a size of a valve opening connecting a line pressure circuitto the converter charge circuit.
 6. A transmission comprising: a firstpump configured to provide fluid to a line pressure circuit in responseto rotation of a transmission input shaft; a second pump driven by anelectric motor; and a switch valve configured to direct fluid from thesecond pump alternately to either the line pressure circuit or to atorque converter charge circuit based on an electric current to anon/off solenoid.
 7. The transmission of claim 6 wherein the switch valvedirects fluid to the line pressure circuit in response to the electriccurrent being less than a threshold and directs fluid to the torqueconverter charge circuit in response to the electric current beinggreater than the threshold.
 8. The transmission of claim 6 furthercomprising: a torque converter defining a torque converter chamber; anda first regulator valve configured to adjust a pressure in the torqueconverter charge circuit based on an electric current supplied to afirst variable force solenoid wherein the torque converter chargecircuit is in fluid communication with the torque converter chamber. 9.The transmission of claim 8 wherein the first regulator valve adjuststhe pressure in the torque converter charge circuit by adjusting a sizeof a valve opening connecting the line pressure circuit to the torqueconverter charge circuit.
 10. The transmission of claim 8 furthercomprising: a torque converter bypass clutch having an apply chamber;and a second regulator valve configured to adjust a pressure in a bypassclutch apply circuit based on the electric current supplied to the firstvariable force solenoid wherein the bypass clutch apply circuit is influid communication with the bypass clutch apply chamber.
 11. Thetransmission of claim 10 further comprising: a third regulator valveconfigured to adjust a pressure in the line pressure circuit based on anelectric current supplied to a second variable force solenoid.
 12. Thetransmission of claim 10 further comprising a lubrication circuit inparallel with the torque converter charge circuit wherein thelubrication circuit is in fluid communication with a balance chamber ofthe bypass clutch.
 13. A transmission control system comprising: a pumpconfigured to supply fluid to a line pressure circuit at a controlledpressure; a converter charge regulator valve configured to adjust a sizeof a valve opening connecting the line pressure circuit to a convertercharge circuit in response to a first electric current; and a flowcontrol valve configured to adjust a flow rate through the convertercharge circuit in response to a second electric current.
 14. The controlsystem of claim 13 further comprising: a bypass clutch regulator valveconfigured to adjust the size of a valve opening connecting the linepressure circuit to a bypass clutch apply circuit in response to thefirst electric current to increase the pressure in the bypass clutchapply circuit as the pressure in the converter charge circuit decreases.